Clostridium difficile infection (CDI) is the leading cause both of hospital-acquired diarrhea and of death due to intestinal infections in the US. The incidence and severity of CDI have increased sharply over the past decade due in part to the emergence and worldwide spread of unusually virulent and deadly strains that overexpress two toxins, A and B. These potent cytotoxins represent the main virulence factors of C. difficile and can damage cells that line the colon, resulting in fluid secretion and inflammation. CDI typically is precipitated by the use of antibiotics, which disrupt the protective colonic flora and provide an opportunity for C. difficile to flourish. Treatment with oral vancomycin or metronidazole leads to symptom resolution in many patients;however, relapse occurs frequently. There is no standard therapy for multiply recurrent CDI or severe, life-threatening disease. Development of new treatment strategies is a global health imperative. One experimental treatment strategy is to neutralize the toxins with monoclonal antibodies (mAbs). Antitoxin mAbs represent a non-antibiotic therapy that is designed to block the harmful effects of the toxins while allowing the normal colonic flora to rebound and the colon to heal naturally. First-generation antitoxin mAbs have shown clinical activity in preventing recurrence of CDI but not in treating active disease. We have generated highly potent humanized mAbs, designated PA-50 and PA-41 that are directed to novel neutralization epitopes on toxins A and B, respectively. Compared with first-generation mAbs, PA-50 and PA-41 are 1,000-fold more potent in neutralizing toxins from virulent BI/NAP1/027 strains in vitro. In a well-established hamster model of CDI, treatment with PA-50/PA-41 provided long-term protection (95% survival) from a lethal challenge, whereas first-generation mAbs delayed but did not prevent death (0% survival). Highly potent mAbs like PA-50 and PA-41 have the potential both to break the cycle of infection in patients with multiply recurrent CDI and to treat life-threatening and otherwise challenging cases of CDI. The overall goal of this project is to complete preclinical studies of our lead mAbs and obtain agreement from FDA to initiate human testing. Initial preclinical studies will delineate the molecular and mechanistic underpinnings of the potent toxin-neutralizing activities of PA-50 and PA-41;these studies will also provide new insights into the biology of C. difficile intoxication. We also will examine the efficacy of the mAbs in treating active disease caused by BI/NAP1/027 strains in a newly established piglet model of CDI. Additional translational activities are designed to complete essential manufacturing, toxicology and other development activities required to prepare and submit Investigational New Drug applications (INDs) for PA-50 and PA-41. Success would ready PA-50 and PA-41 for human clinical testing and would constitute a key milestone in the development of innovative new agents that may address key challenges of the C. difficile epidemic. PUBLIC HEALTH RELEVANCE: Clostridium difficile is a spore-forming bacterium that represents the leading cause of hospital-acquired diarrhea and affects approximately 500,000 individuals annually in the US. Unusually deadly strains have emerged in recent years, and new treatments are urgently needed. C. difficile infection (CDI) typically strikes individuals who are taking antibiotics, which can disrupt the protective gut microbes and provide an opportunity for C. difficile to flourish. C. difficile produces toxins that can damage the cells that line the large intestines. One experimental treatment strategy is to block with harmful effects of the toxins with monoclonal antibodies. We have developed novel antibodies that potently neutralize C. difficile toxins in preclinical models of CDI. In this project, we seek to complete studies that will ready our antitoxin antibodies for clinical testing as non- antibiotic therapies for CDI.